Lyme disease (Steere, A. C., Proc. Natl. Acad Sci. USA 91:2378-2383 (1991)), or Lyme borreliosis, is presently the most common human disease in the United States transmitted by an arthropod vector (Center for Disease Control, Morbid Mortal. Week Rep. 46(23):531-535 (1997)). Further, infection of house-hold pets, such as dogs, is a considerable problem.
While initial symptoms often include a rash at the infection point, Lyme disease is a multisystemic disorder that may include arthritic, cardiac, and neurological manifestations. While antibiotics are currently used to treat active cases of Lyme disease, B. burgdorferi persists even after prolonged antibiotic treatment. Further, B. burgdorferi can persist for years in a mammalian host in the presence of an active immune response (Straubinger. R. et al., J. Clin. Microbiol. 35:111-116 (1997); Steere, A., N. Engl. J. Med. 321:586-596 (1989)).
Lyme disease is caused by the related tick-borne-spirochetes classified as Borrelia burgdorferi sensu lato (including B. burgdorferi sensu stricto, B. afzelii, B. garinii). Although substantial progress has been made in the biochemical, ultrastructural, and genetic characterization of the organism; the spirochetal factors responsible for infectivity, immune evasion and disease pathogenesis remain largely obscure.
A number of antigenic B. burgdorferi cell surface proteins have been identified. These include the outer membrane surface proteins (Osp) OspA, OspB, OspC and OspD. OspA and OspB are encoded by tightly linked tandem genes which am transcribed as a single transcriptional unit (Brusca, J. et at, J. Bacteriol. 173:8004-8008 (1991)). The most-studied B. burgdorferi membrane protein is OspA, a lipoprotein antigen expressed by borreliae in testing ticks and the most abundant protein expressed in vitro by most borrelial isolates (Barbour, A. G., et al., Infection & Immunity 41:795-804 (1983); Howe, T. R. et al., Science 227:645 (1985)).
A number of different types of Lyme disease vaccines have been shown to induce immunological responses. Whole-cell B. burgdorferi vaccines, for example, have been shown to induce both immunological responses and protective immunity in several animal models (Reviewed in Wormser, G., Clin. Infect. Dis. 21:1267-1274 (1995)). Further, passive immunity has been demonstrated in both humans and other animals using B. burgdorferi specific antisera.
While whole-cell Lyme disease vaccines confer protective immunity in animal models, use of such vaccines presents the risk that responsive antibodies will produce an autoimmune response (Reviewed in Wormser, G., supra). This problem is at least partly the result of the production of B. burgdorferi specific antibodies which cross-react with hepatocytes and both muscle and nerve cells. B. burgdorferi heat shock proteins and the 41-kd flagellin subunit are believed to contain antigens which elicit production of these cross-reactive antibodies.
Single protein subunit vaccines for Lyme disease have also been-tested. The cell surface proteins of B. burgdorferi are potential candidates for use in such vaccines and several have been shown to elicit protective immune responses in mammals (Probert, W. et al., Vaccine 15:15-19 (1997); Fikrig, E. er al., Infect. Immun. 63: 1658-1662 (1995); Langerman S. et al., Nature 372:552-556 (1994); Fikrig, E. et al., J. Immunol. 148:2256-2260 (1992)). Experimental OspA vaccines, for example, have demonstrated efficacy in several animal models (Fikrig, E., et al., Proc. Natl. Acad. Sci. USA 89:5418-5421 (1992); Johnson, B. J., et al., Vaccine 13:1036-1094 (1996); Fikrig, E., et al., Infect. Immun. 60:657-661 (1992); Chang, Y. F., et al., Infection & Immunity 63:3543-3549 (1995)), and OspA vaccines for human use are under clinical evaluation (Keller, D., et al., J. Am. Med. Assoc. 271:1764-1768 (1994); Van Hoecke, C., et al., Vaccine 14:1620-1626 (1996)). Passive immunity is also conferred by antisera containing antibodies specific for the full-length OspA protein. Further, vaccination with plasmid DNA encoding OspA has been demonstrated to elicit protective immune responses in mice (Luke, C. et al., J. Infect. Dis. 175:91-97 (1997); Zhong, W. et al., Eur. J. Immunol. 26:2749-2757 (1996)).
Recent immunofluorescence assay observations indicate that during tick engorgement the expression of OspA by borreliae diminishes (deSilva, A. M., et al., J. Exp. Med. 183:271-275 (1996)) while expression of other proteins, exemplified by OspC, increases (Schwan, T. G., et al., Proc. Natl. Acad. Sci. USA 92:2909-2913 (1985)). By the time of transmission to hosts, spirochetes in the tick salivary glands express little or no OspA. This down-modulation of OspA appears to explain the difficulties in demonstrating immune responses to this antigen early in infection following tick bites (Kalish, R. A., et al., Infect. Immun. 63:2228-2235 (1995); Gem, L., et al., J. Infect. Dis. 167:971-975 (1993); Schiable, U. E., et al., Immunol. Lett. 36:219-226 (1993)) or following challenge with limiting doses of cultured borreliae (Schiable, U. E., et al., Immunol. Lett. 36:219-226 (1993); Barthold, S. W. and Bockenstedt, L. K., Infect. Immun. 61:4696-4702 (1993)).
Furthermore, OspA-specific antibodies are ineffective if administered after a borrelial challenge delivered by syringe (Schiable, U. E., et al., Proc. Natl. Acad. Sci. USA 87:3768-3772 (1990)) or tick bite (deSilva, A. M., et al., J. Exp. Med. 183:271-275 (1996)). To be efficacious, OspA vaccines must elicit protective levels of antibody which are maintained throughout periods of tick exposure in order to block borrelia transmission from the arthropod vector.
Vaccines in current use against other pathogens include in vivo-expressed antigens which could boost anamnestic responses upon infection, potentiate the action of immune effector cells and complement, and inhibit key virulence mechanisms. OspC is both expressed during infection (Montgomery, R. R., et al., J. Exp. Med. 183:261-269 (1996)) and a target for protective immunity (Gilmore, R. D., et al., Infect Immun. 64:2234-2239 (1996); Probert, W. S. and LeFebvre, R. B., Infect. Immun. 62: 1920-1926 (1994); Preac-Mursic, V., et. al., Infection 20:342-349 (1992)), but mice immunized with this protein were only protected against challenge with the homologous borrelial isolate (Probert, W. S., et al., J. Infect Dis. 175:400-405 (1997)). Identification of in vivo-expressed, and broadly protective, antigens of B. burgdorferi has remained elusive.